Dye stripper

Right, kids. I have much-dyed black hair. I also have some bright blue dye.
I want to put in some bleached stripes then put this blue in over that.
Yes, I am aware that it could go horribly wrong, but I'm going to take my chances. The question is, where do I go to find a) dye stripper and b) bleach/peroxide stuff? Is Boots going to work or am I going to have to go further afield?